The search for economical methods to produce silicon for photovoltaic applications has been ongoing for over three decades. The sources of raw materials for these methods have been largely limited to silica in the form of quartz and volatile compounds of silicon, with limited examination of rice husk as the source of silicon. The processing methods that have received the most research attention have been the upgrading of metallurgical grade silicon by modification of conventional production methods and by pyrolytic decomposition of halosilanes i.e. the so-called Siemens Process. These two methods are commonly referred to as the metallurgical and chemical routes to solar grade silicon (SoG-Si) synthesis.
Although the chemical route has been used successfully to meet and exceed the typical purity requirements for SoG-Si, the potential for lowering costs and increasing the volume of production has been very limited. The unit processing steps involved in the chemical route approach can be complex, energy intensive and of long duration, as is discussed by Braga A. F. B, Moreira S. P, Zampieri P. R, Bacchin J. M. G, Mei P. R: “New Processes for the Production of Solar grade Polycrystalline Silicon—A Review”, Solar Energy Materials and Solar Cells, Vol. 92(2008) pp 418-424.
A metallurgical route combined with directional solidification has been successful in removing metallic impurities with low segregation coefficients from silicon. However, this route has been ineffective economically in removing high segregation impurities e.g. B, P, Al. The latter are known to be very deleterious to the performance of solar cells, as is discussed in Istratov A. A, Buonassisi T, Pickett M. D, Heuer M, Weber E. R. “Control of impurities in “dirty” multicrystalline silicon for solar cells” Materials Science and Engineering B 134(2006) Elsevier B. V. pp 282-286. Thus, the logical and consequently typical approach has been to use very high purity silica and reductant raw materials that are almost free of high segregation impurities.
Rice husk is a waste by-product of the rice milling industry. The rice husk constitutes about 20% by weight of a rice paddy. The mineral ash content is 15-30% of the rice husks, of which 87-97% is amorphous silica. An estimated eighty million (80×106) metric tonnes of rice husk are generated worldwide annually. The economic potential of utilizing this large amount of waste rice husk has attracted several research interests in recent years, including processing rice husk into other value added products e.g. as purified silica, carbides and nitrides of silicon, and insulating material in the steel industries.
A method of obtaining silicon of 6N (99.9999%) purity by reducing white rice husk ash with magnesium at a temperature of 800° C. followed by several successive acid leaching treatments is reported by Singh Rajvir and Dhindaw B. K, “Production of High Purity Silicon for Use in Solar Cells” Sun, Mankind's Future Source of Energy. edited by Francis de Winter and Michael Cox, Vol. II (1978), pp 776-781, Pergamon Press 1978, authored by International Solar Energy Society (ISES). The possibility of obtaining silicon of similar purity by direct smelting of purified amorphous silica with carbonaceous reductants in an electric furnace followed by leaching with acids, and repeating the steps about nine times, was also suggested. The method used to analyze the 6N silicon was not reported. However, the cost of such repeated smelting and leaching would be expected to prohibit use of this method as a low cost alternative to conventional methods.
U.S. Pat. No. 4,214,920 of Amick et al describes a method for producing high purity silicon from rice husks by coking leached rice hulls, adjusting the carbon:silica ratio to 2:1 followed by thermal reduction. Hunt, L. P. Dismukes, J. P. Amick, J. A. Schei, A. and Larsen, K. “Rice Hulls as a Raw Material for Producing Silicon” J. Electrochem. Soc. 131, No. 7 (1984.) pp 1683-1686 investigated the possibility of producing high purity silicon from rice husk by purifying rice husk silica according to the above method of Amick et al, followed by pelletizing and reduction in a modified electric arc furnace. The pelletizing was carried out using carbon black as a reductant and sucrose as a binder. Modifying the electric arc furnace was essential to minimize or eliminate possible contamination from the furnace atmosphere.
Bose D. N, Govindacharyulu P. A, Barnejee H. D “Large Grain Polycrystalline Silicon from Rice husk, Solar Energy Materials, Vol. 7 (1982) North Holland Publishing Company pp 319-321 subjected powdered silicon obtained by magnesium reduction of rice husk ash to melting and directional solidification. It was found that boron was the active impurity in the polycrystalline silicon ingot that was obtained. It was also determined that the minority carrier life time of the polycrystalline silicon material was of the order of 1-5 μs, and thus promising for photovoltaic applications. However, it has been subsequently estimated that the minimum carrier lifetime requirement for efficient solar cells fabricated from multicrystalline silicon wafers is 25 μs. The formation of crystalline silicon by heating a silicon precursor e.g. silicon dioxide, with an ingredient that will generate an exothermic reaction when heated e.g. magnesium, and isolating crystalline silicon is described in US published application 2009/0010833 of Rosenband V. et al, published 8 Jan. 2009.
The magnesium reduction of rice husk ash has also been reported by Banerjee H. D, Sen S., Acharya H. N: “Investigations on the Production of Silicon from Rice Husk by the Magnesium Method”, Materials Science and Engineering, 52 (1982) pp 173-179. Acid leached rice husk ash was reduced by a method involving intimately mixing the ash with magnesium powder and firing the powdered mixture at temperatures between 500-600° C. in a sealed graphite crucible in a muffle furnace. The reaction product was successively leached in mineral acids (HCl, H2SO4, and HF) in a Teflon™ beaker. Some degree of crystallinity in the muffle furnace-fired rice husk silica was reported. Spectrochemical analysis of the final silicon product showed high contents of boron (20-200 ppm), magnesium (50-1000 ppm) and aluminum (10-200 ppm). The contamination of the silicon was attributed to the use of laboratory grade magnesium and laboratory glassware. Nazma Ikram, and Akhter M, “XRD Analysis of Silicon Prepared from Rice Husk Ash”, Journal of Materials Science, vol 23 (1988), pp 2379-2381 reported a similar approach but using 4N purity magnesium; the silicon obtained was of 99.95% purity with a boron content of approximately 2 ppm. It was concluded that the silicon could be upgraded to solar grade silicon by conventional refining methods.
Calciothermic reduction of purified rice husk ash was reported by Mishra P, Chakraverty A., Banerjee H. D; Production and Purification of Silicon by Calcium Reduction of Rice Husk White Ash, Journal of Materials Science, vol 20 (1985) pp 4387-4391. A stoichiometric composition of granular calcium and purified rice husk silica was mixed, and the powdered mixture obtained was fired in a sealed sillimanite crucible in a muffle furnace at a temperature of about 720° C. The reduction product was milled to fine powder and successively leached with concentrated nitric acid (HNO3) and hydrofluoric acid (HF) to obtain silicon of 99.9% purity with boron content of 10 ppm. It was suggested that the use of MgO-coated crucibles and high purity reagent could lead to the production of solar grade silicon by this method.
Silica fume is a byproduct of the silicon and ferrosilicon production industry. It is high purity silica (+90%) in the amorphous form. Use of silica fume as the feedstock for solar grade silicon synthesis has not been reported.
With respect to operation of economical commercial-scale processes for the production of solar grade silicon, it is believed that improvements are required in the methods described above. For instance, combustion of rice husk has been generally carried out in reactors with high temperature and long residence time, which results in formation of crystalline silica in the ash, which is a known carcinogen. Therefore, treatment or use of the ash from the conventional processes is limited.
A process for the synthesis of high purity silicon, including so-called SoG-Si, with high volume of production at low cost would be desirable.